Squaraine

Article pubs.acs.org/IECR

Fabrication and Performance of Copper Phthalocyanine/Squaraine Dye/Perylene Composite with Bulk Heterojunctions by the Solution Blending Process Chien-Hsun Chen,† Wen-Tung Cheng,†,* Ming-Liao Tsai,‡ and Kou-Tung Huang§ †

Department of Chemical Engineering, National Chung Hsing University, Taichung 40227, Taiwan, Republic of China Department of Chemical Engineering, National Chin-Yi University of Technology, Taichung 411, Taiwan, Republic of China § Material and Chemical Research Laboratory, Industrial Technology of Research Institute, Hsinchu, 310, Taiwan, Republic of China ‡

ABSTRACT: In this paper, copper tetra-(2, 4-dimethyl-3-pentoxy) phthalocyanine (CuPc-712), 2-[2-methylene-1,1,3-trimethyl1H-benzo[e]inline]-4-[1-ethanol-2-methylquino]-cyclobutadienylium-1,3-diolate (SQ-700), and 3,4,9,10-tetra-(heptyl acetate)perylene (Pery-C7), attached with functional groups by covalent bonds were synthesized. In addition, bulk heterojunction diodes made from ITO/PEDOT:PSS (CuPc-712+SQ-700+Pery-C7)/Al have been realized and studied by I−V characteristic measurements in the dark and under illumination (ITO is indium tin oxide; PEDOT:PSS is poly(3,4-ethylene dioxythiophene) oxidized (doped) with poly(4-styrene sulfonate)). Enhanced photosensitivity was observed in the photoreceptor made from the composite (CuPc-712+SQ-700+Pery-C7), which was interpreted in terms of the expansion of light absorption spectrum and to charge separation by the bulk heterojunctions. In addition, the presence of a thin PEDOT:PSS film at the ITO/(CuPc-712+SQ-700+Pery-C7) interface allows one to achieve a significant photovoltaic efficiency of ∼0.2%. squaric acid.10 Furthermore, perylene is an n-type (electron acceptor) organic material. These components are difficult to dissolve using a solvent, because of the symmetric structure of the molecule.11 In this research, a solution blending process is employed to fabricate the D/A composite with bulk heterojunctions from soluble copper tetra-(2,4-dimethyl-3-pentoxy)phthalocyanine (named as CuPc-712; see Figure 1) and 2-[2-methylene-1,1,3-

1. INTRODUCTION For the renewable energy, many scientists have engaged in efforts to research the concept of bulk heterojunctions, which has been introduced to fabricate organic photovoltaic solar cells.1,2 The organic photovoltaic device is chiefly attributed to the introduction of the donor/acceptor (D/A) heterojunction that functions as a dissociation site for the strongly bound photogenerated excitons.3−8 It was further realized by using blends of the donor and acceptor materials and found that phase separation during spin coating led a bulk heterojunction that removed the exciton diffusion bottleneck by creating an interpenetrating network of the donor and acceptor materials. Such blends allow some efficiency in the fundamental steps necessary to achieve significant organic photovoltaic efficiency. For an organic solar cell, the overall power conversion efficiencies are dominated by the following steps: (1) photoabsorption and excition generation; (2) exciton diffusion to the D/A interface; (3) exciton split or charged carrier generation at the D/A interface; (4) carrier diffusion to the respective electrodes; and (5) carrier collection by the electrodes. Compared to the hybrid and inorganic solar cell, the organicbased photovoltaic efficiency is low, but it is very attractive for low-cost, lightweight, large-area, and flexible-shaped solar sheets. Phthalocyanine is a macrocyclic compound with four isoindole-class [(C6H4)C2N] units linked by four N atoms to form a conjugated chain, which participate in hosting various different metal ions in its center, the macrocyclic structure of which shows a striking feature as a colorant like porphyrins (biopigments) in nature,9 and squaraine is a class of organic dyes with very intense fluorescence spectra, typically in the red and near-infrared (NIR) region (absorption maxima are found between 630 and 670 nm, and their emission maxima are between 650 and 700 nm), which is characterized by their unique aromatic four-membered ring system derived from © 2012 American Chemical Society

Figure 1. Chemical structure of CuPc-712.

trimethyl-1H-benzo[e]indoline]-4-[1-ethanol-2-methylquino]cyclobutadienylium-1,3-diolate (named as SQ-700; see Figure 2), where both are used as the electron donor, as well as 3,4,9,10tetra-(heptyl acetate)-perylene (named as Pery-C7; see Figure 3), which is used as the electron acceptor. Bulk heterojunction diodes made using ITO/PEDOT:PSS/(CuPc-712+SQ-700 Received: Revised: Accepted: Published: 3630

July 22, 2011 January 27, 2012 February 11, 2012 February 12, 2012 dx.doi.org/10.1021/ie201586b | Ind. Eng. Chem. Res. 2012, 51, 3630−3638

Industrial & Engineering Chemistry Research

Article

Perkin−Elmer Model 2400C elemental analysis system. The morphology and thickness of the cast film were observed using a JEOL Model JSM-6700F FESEM instrument. A cyclic voltammeter (CHI Instruments, Model 6272C) was used to measure the oxidation reduction potential, where Pt was used as the working electrode and Ag/AgCl was used as the reference electrode. The organic electrolyte of anhydrous DMF includes 0.1 M tetrabutylammonium perchlorate (TBAP), which is scanned at a rate of 100 mV/s. For the I−V measurements, the ITO/blend organic composite/Al sandwich structures were used. The I−V curves of the cells were measured in air at ambient temperature, using a Lambda power supply as the source voltage and a Keithley apparatus. The curves were recorded in dark-field mode under AM1.5 illumination, using an Oriel solar simulator that was equipped with filters (P = 100 mW/cm2). 2.3. Synthesis of Copper tetra-(2,4-dimethyl-3pentoxy)phthalocyanine (CuPc-712) (Scheme 1). Following the literature,12−14 3-nitrophthalonitrile (77 mmol) and 2,4dimethyl-3-pentanol (6.93 mmol) was dissolved in the dry N,N′-dimethylformamide (DMF) solution of 10 mL, and then K2CO3 (7.5 mmol) was added at ambient temperature. The heating temperature is 140 °C, and the reaction requires 4 h. After the reaction, the solution was added into water (100 mL) and stirred for 1 h. The solid precipitate of 3-(2,4-dimethyl-3pentoxy)phthalodinitrile (1) was collected with filter paper. 1H NMR (200 MHz/CDCl3), ppm δ: 0.97 (dd, J = 12.4 Hz, J = 6.8 Hz, 12H), 2.04−2.20 (m, 2H), 4.0 (t, J = 5.8 Hz, 1H), 7.25 (d, J = 1,2 Hz, 2H), and 7.58 (t, J = 8.4 Hz, 1H). Compound (1) (8.26 mmol) mixes with urea (16.5 mmol), ammonium molybdate (0.2 mmol), copper(I) chloride (CuCl) (2 mmol), and nitrobenzene (8 mL), and then it was heated at 160 °C for 6 h. After purification, the solid part can be dried in a vacuum drying machine at 60 °C overnight, and blue-green powder of copper tetra-(2,4-dimethyl-3-pentoxy) phthalocyanine (CuPc-712) was finally obtained; the maximum absorbing wavelength in the UV−Vis spectra centered at 712 nm (CHCl3) and FT-IR (KBr) was located at 3064.4, 2960.3, 2929.9, 2879.5, 1587.0, 1485.1, 1267.0, 1121.9, and 1083.1 cm−1. Anal. Calcd for (C60H72CuN8O4): C, 69.77; H, 7.03; and N, 10.85. Found: C, 69.18; H, 6.83; and N, 10.84, respectively. 2.4. Synthesis of 2-[2-methylene-1,1,3-trimethyl-1Hbenzo[e]indoline]-4-[1-ethanol-2-methylquino]cyclobutadienylium-1,3-diolate (SQ-700) (Scheme 2). Based on the reports,15−17 a mixture of 2,3,3-trimethyl-1Hbenzo[e]indole (14.3 mmol) and methyl iodide (15.7 mmol) was refluxed for 8 h in ethyl acetate (20 mL). A solid precipitate of 3-tetramethyl-1H-benzo[e]indoleium iodide (2) was then

Figure 2. Chemical structure of SQ-700.

Figure 3. Chemical structure of Pery-C7.

+Pery-C7)/Al also have been realized and studied by current− voltage (I−V) characteristic measurements in darkness and under illumination (ITO is indium tin oxide; PEDOT is PSS: poly(3,4-ethylene dioxythiophene) oxidized (doped) with poly(4-styrene sulfonate)).

2. EXPERIMENTAL SECTION 2.1. Materials. 3-Nitrophthalonitrile and 2,4-dimethyl-3pentanol were manufactured by Tokyo Chemical Industry Co. Ammonium molybdate, nitrobenzene 2,3,3-trimethyl-1H-benzo[e]indole, methyl iodide, 2-methylquinoline, 2-bromomethanol, 2-bromoethanol, perylene-3,4,9,10-tetracarboxylic acid dianhydride, tetraoctyl ammonium bromide, and 1-bromoheptane were purchased from Acros Chemical Co. N,N-dimethylformamide (DMF), methanol, ethyl acetate, dichloromethane, n-butanol, pyridine, acetonitrile, and toluene were obtained from Tedia Chemical Co. Potassium carbonate, urea, and copper(I) chloride (CuCl) were fabricated by Showa Chemical Industry Co. In addition, PEDOT:PSS (polyethylene dioxythiophene: poly(4styrene sulfonate)) was manufactured by Bayer. These chemical materials had no further purification. All solvents were analytical grade and were distilled before use. 2.2. Characterization. Ultraviolet-visible light (UV−Vis) absorption spectra of the cast film were analyzed using a Perkin−Elmer Lambda Type 900 system. Fourier Transform infrared (FTIR) spectrometric analysis was performed using a Perkin−Elmer Model One B spectrometer to determine the functional groups. Thermogravimetric analysis (TGA) was performed using a Perkin−Elmer thermogravimetric analyzer (Pyris:1 TGA), the heating rate of which was 10 °C/min in a N2 environment. 1H nuclear magnetic resonance (NMR) spectra was recorded using a Varian Oxford FT-NMR spectrometer. The C, H, and N elemental analysis was performed using a Scheme 1. Synthesis Route of CuPc-712

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dx.doi.org/10.1021/ie201586b | Ind. Eng. Chem. Res. 2012, 51, 3630−3638


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Scheme 2. Synthesis Route of SQ-700

and butanol (50 mL), followed by dehydration at 125 °C for 14 h. After purification, the crystals precipitate of 2-[2methylene-1,1,3-trimethyl-1H-benzo[e]indoline]-4-[1-ethanol-2methylquino]cyclobutadienylium-1,3-diolate (SQ-700) was produced. FT-IR (KBr) υ = 3400.3, 2924.3, 1560.7, 1500.6, 1291.4, 1121.8 cm−1 ; 1H NMR (DMSO-d6), ppm δ: 1.66 (s, 6H) 2.72 (s, 3H), 3.07 (t, J = 3.6 Hz, 2H), 3.76 (t, J = 4.1 Hz, 2H), 4.74 (s, 1H), 5.20 (s, 1H) 6.63−8.62 (m, 12H). Anal. Calcd for SQ-700(C32H28N2O3): C, 78.67; H, 5.73; N, 5.73. Found: C, 78.18; H, 5.99; and N, 5.73. 2.5. Synthesis of 3,4,9,10-tetra-(heptyl acetate)perylene(Pery-C7) (Scheme 3). According to the researchs,18−20 perylene-3,4,9,10-tetracarboxylic acid dianhydride

obtained after filtration. 1H NMR (200 MHz/CDCl3), ppm δ: 1. 75 (s, 6H), 3.46 (s, 3H), 4.13 (s, 3H), and 7.50−8.0 (m, 6H). 2-Methylquinoline (20 mmol), 2-bromoethanol (24 mmol), and ethyl acetate (20 mL) are mixed and reacted at 90 °C for 16 h. The solid precipitation of 1-ethanol-2-methylquinolbromide (3) was given after purification. 1H NMR (200 MHz/ DMSO): 3.20 (s, 3H), 3.34 (s, OH), 3.98 (t, J = 5.2 Hz, 2H),5.14 (t, J = 5.4 Hz, 2H), 7.92−8.38 (m, 3H), 8.41 (d, J = 8 Hz, 1H), 8.63 (d,J = 9.2 Hz, 1H), and 9.12 (d,J = 8.6 Hz, 1H). The mixture of 3,4-dihydroxy-3-cyclobutene-1,2-dione (2 g 17.5 mmol), n-butanol (10 mL), and toluene (10 mL) was reacted at 125 °C for 14 h to dehydrate. After the solvent was removed, an oily product of 3,4-dibutoxycyclobutane-1,2-dione (4) was collected and applied in the next step without further purification. 1H NMR (200 MHz/CDCl3), ppm δ: 0.91 (t, J = 7 Hz, 6H), 1.37−1.53 (m, 4H), 1.70−1.84 (m, 4H), and 4.66 (t, J = 6.6 Hz, 4H). Compound (2) (3.4 mmol), an intermediate (4) (5 mmol), and K2CO3 (5 mmol), along with MeOH (10 mL), were mixed to react at room temperature for 14 h. After purification, the product of 2-[2-methylene-1,1,3-trimethyl-1H-benzo[e]indole]-3-butoxyl-cyclobutene-1,2-dione (5) was obtained. 1H NMR (200 MHz/CDCl3), ppm δ: 1.02 (t, J = 7.4 Hz, 3H), 1.44−1.51 (m, 4H), 1.99 (s, 6H), 3.48 (s, 3H), 4.85 (t, J = 7 Hz, 2H), 5.40 (s, 1H), and 7.26−8.71 (m, 6H). The compound (5) (31.3 mmol) was added into CH3CN (58.75 mL), followed by slow dropwise addition into 37% HCl (29.375 mL) at room temperature until completely dissolved. After purification, the product of 2-[2-methylene-,1,1,3trimethyl-1-1H-benzo[e]indole]-3-hydroxyl-cyclobutene-1,2dione (6) was obtained. 1H NMR (200 MHz/CDCl3), ppm δ: 1.99 (s, 6H), 3.0 (s, 3H), 3.49 (s, OH), 5.42 (s, 1H), and 7.26−8.71 (m, 6H). Finally, the compounds 6 (5 g, 15 mmol) and 3 (4 g, 15 mmol) were mixed with pyridine (1.42 g), toluene (50 mL),

Scheme 3. Synthesis Route of Pery-C7

(25 mmol) was charged into KOH (0.1 mol/L 10 mL) and heated to 90 °C, followed by adding tetraoctyl ammonium bromide (TOAB)(0.2 g) and 1-bromoheptane (10 mmol) to react for 6 h. After draining the solvent and subjecting the mixture to crystallization, the 3,4,9,10-tetra-(heptyl acetate)perylene(Pery-C7) was formed, which FT-IR (KBr) υ = 2951.8, 2929.3, 2862.9, 1731.3, 1713.2, 1269.9, and 1167.4 cm−1; and 1H NMR (200 MHz/CDCl3), ppm δ: 0.89 (t, J = 6.6 Hz, 12H), 3632



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Therefore, there are 2H and 12H, on δ = 2.24 and 2.98 ppm, respectively, for compound (1). Fabricating CuPc-712 requires the addition of CuCl at high temperature. Meanwhile, a small amount of ammonium molybdate is necessary for catalyzation during reaction. The appearance of the product becomes dark green after the reaction. The structure of CuPc-712 contains ionic copper, so it cannot be measured via 1H NMR. As observed from FTIR analysis (Figure 4), the CN group on 2230 cm−1 disappears when CuPc-712 is formed. The featured functional groups have been found at 3064 cm−1 (Ar−H stretch); 2960, 2992, and 2879 cm−1 (aliphatic CH stretch); 1587 and 14859 cm−1 (Ar CC stretch); 1267 cm−1 (Ar−O−Ar stretch); and 1221 cm−1 (ester C−O stretch). This new compound can be dissolved by the organic solvents such as DMF, toluene, ethyl acetate, acetone, dichloromethane, and chloroform. Compound, 1,1,2,3-tetramethyl-1H-benzo[e]indolinium iodide (2) and 4-dibutoxycyclobutane-1,2-dione (4) were reacted unilaterally, replacing 2-[2-methylene-1,1,3-trimethyl-1Hbenzo[e]indole]-3-butoxyl-cyclobutene-1,2-dione (5); patronation was then provided by hydrochloric acid (HCl) to obtain 2-[2-methylene-1,1,3-trimethyl-1H-benzo[e]indole]-3-hydroxyl-cyclobutene-1,2-dione (6); finally, 1-ethanol 2-methylquinol bromide (3) was added to obtained the required asymmetric squaric acid dye, 2-[2-methylene-1,1,3-trimethyl-1H-benzo[e]indoline]-4-[1-ethanol-2-methylquino]cyclobutadienylium-1,3diolate (SQ-700). According to the FTIR analysis by Treibs and Jacob,24 as shown in Figure 5a, the 1,2-type of squaric acid

1.30−1.83 (m, 40H), 4.32 (t, J = 6.2 Hz, 8H), 7.99 (d, J = 8.0 Hz, 4H), and 8.21 (d, J = 8 Hz, 4H). Anal. Calcd for Pery-C7 (C52H68O8): C, 76.06; and H, 8.35. Found: C, 75.62; and H, 8.34. 2.6. Fabrication of Cells. The composite was obtained by mixing the CH2Cl2/toulene (0.5 g/1.5 g) solutions containing the components of CuPc-C7, SQ-700, and Pery-C7. The proportions of the three as-prepared components are weight ratios for cell fabrication. The composite films were made by the casting method on a transparent conductive oxide (TCO)-coated glass. The TCO used was commercial indium tin oxide (ITO) (provided by Ritek, Taiwan). The entire glass substrate was covered; therefore, some ITO must be removed to obtain the under electrode. After masking a broad line of 4 mm, the ITO was etched by using proprietary enchants.21 The substrates then were cleaned, using the H2O2 treatment, following a process described by Osada et al.,22 which corresponds to the first solution (SC1) of the RCA process.23 The substrates were treated with a H2O−H2O2 (30%)−NH4OH (25%) solution (5:1:1 in volume parts) at 80 °C for 20 min, followed by rinsing with boiling distilled H2O for 5 min. The use of boiling water was proven to be helpful to obtain impurity-free surfaces. In the case of ITO/PEDOT:PSS/(CuPc-712+SQ-700+PeryC7)/Al structures, the PEDOT:PSS was spin-coated onto the ITO and heated at 120 °C for 2 h. PEDOT:PSS was spincoated because it was in aqueous dispersion with H2O as a pristine solvent. Finally, Al upper contacts were deposited onto the organic layer via vacuum evaporation at low pressure (close to 10−4 Pa). A mask was used to determine a well shape for the Al electrode, which gives an active area of 2 mm2.

3. RESULTS AND DISCUSSION 3.1. Analyses of NMR and FTIR Analyses. 3-(2,4dimethylpentan-3-yl-oxy)benzene-1,2-dinitrile (1) was synthesized by utilizing the alkaline electrophilic substituent to replace the nitro in the benzene ring in K2CO3/DMF at 140 °C. As analyzed with FTIR (Figure 4), the −NO2 in the benzene ring

Figure 5. Chemical structures of (a) the 1,2-type and (b) the 1,3-type of squaraine acid.

has two groups of CO, the wavenumber of which is 1700− 1800 cm−1, whereas the 1,3-type is basically a tetracycline structure (C4O2) with a wavenumber ranging from 1560 to 1620 cm−1, which strong signal located at 1600 cm−1. Therefore, the as-synthesized SQ-700 is a 1,3-type one, as shown in Figure 5b. This means that the signal of hydrogen for carbon on the tetracycline with δ = 4.74 and 5.20 ppm is a very important index to determine SQ-700. Synthesizing Pery-C7 requires a phase transform agent. PTCDA salts were solved in water. 1-Bromoheptane is another starter of organic material, which is uanble to be dissolved in water. Both are difficult to react. Therefore, a phase transform agent is needed to enhance the reaction. Tetraoctyl ammonium bromide (TOAB) is a standard phase transform agent, and it can be dissolved in a water phase. In addition, it has a pair of lipophilics that can be dissolved in an organic phase for the replacement reaction, increasing the reaction speed to obtain products. In FTIR spectra (Figure 4), the CO stretch of ester can be observed, the wavenumber of which (1731 cm−1) indicates that the product has formed.

Figure 4. Infrared (IR) spectra for CuPc-712, SQ-700, and Pery-C7.

located at 1546 cm−1 disappears, but a new functional group (C−O−C) centered at 1246 cm−1 is generated. Furthermore, for the benzene ring of 1HNMR, hydrogen is indicated by δ = 7.25−7.58 ppm and phenyl ether (ROC6H6) is symmetric. 3633



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3.2. Absorption Spectra of Cast-Coated Films. The absorption spectra of CuPc-712 is dominated by the Q-band and the B-band; the UV-Vis wavelength on the Q-band is 600− 800 nm and that on the B-band is 300−400 nm.25 As shown in Figure 6a, in CHCl3 solution, the maximum absorbing peaks of

Figure 7. Absorption spectra on cast film made from (a) CuPc712+Pery-C7 (1:4) (solid line, ), (b) SQ-700+Pery-C7 (1:4) (dashed line, − − −), and (c) CuPc-712+SQ-700+Pery-C7 (1:1:4) (dashed-dotted line, − · −).

Figure 8. Scanning electron microscopy (SEM) images of cast film made from (a) CuPc-712, (b) SQ-700, and (c) Pery-C7. Figure 6. Absorption spectra of (a) solution and (b) cast film made from CuPc-712 (solid line, ), SQ-700 (dashed line, − − −), and Pery-C7 (dashed-dotted line, − · −), respectively.

the UV-Vis irradiation of CuPc-712 and SQ-700 are centered at 712 and 700 nm, respectively; Pery-C7 also has two absorbing peaks, which are located at 471 and 443 nm, respectively. For the cast film, its aborbing peak has a red shift, forming a wider absorbing peak, as shown in Figure 6b. The main reason is the increment of concentration causing molecule congregation, namely, the J-aggregate.26 The use of solvent-blended CuPc712/Pery-C7 (1:4), SQ-700/Pery-C7 (1:4), CuPc-712/SQ700/Pery-C7 (1:1:4), respectively, for cast film to increase the absorbing range of UV-Vis irradiation in solar spectra generating more excitors, as illustrated in Figure 7. 3.3. Scanning Electron Microscopy (SEM) of the CastCoated Film. The morphologies of as-synthesized compounds are shown in Figure 8. Figure 8a represents pure CuPc-712, and there are many small piles distributed on the glass substrate; Figure 8b represents pure SQ-700, and it shows piles of flakes; and Figure 8c represents pure Pery-C7, and it displays regular strips. The evolution of the surface morphology of the cast films from the blended solutions containing different contents of CuPc-712, SQ-700, and Pery-C7 can be seen via SEM, as shown in Figure 9. The film morphology of the composite is quite different. The film made from the CuPc-712/Pery-C7 composite with a 1:4 weight ratio appears more compact than that of SQ-700/Pery-C7, and the sizes of the apertures in the

Figure 9. SEM images of cast film made from (a) CuPc-712+Pery-C7 (1:4), (b) SQ-700+Pery-C7 (1:4), and (c) CuPc-712+SQ-700+PeryC7 (1:1:4).

film also are smaller (see Figures 9a and 9b). The interesting thing is that small and uniform grains are found in the composite film made from three components of CuPc-712, SQ700, and Pery-C7 with the 1:1:4 in weight ratios (Figure 9c). Such an evolution of the film morphology suggests that a bulk heterojunction might exist in the composite (CuPc-712+SQ700+Pery-C7) with suitable proportion.11 It can be explained that, during cast-coating and solvent evaporation, the phase separation is induced by the interactions of CuPc-712, SQ-700, and Pery-C7 molecules, creating an intricate interpenetrating network. It is expected that, after photon absorption and exciton creation, the exciton should be dissociated at the large D/A interfacial area created by the blend structure, which is favorable to the dissociation of excitons and is promising for organic optoelectronic devices. 3634



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As observed from Figure 10, the visualization of a crosssection of a structure glass/ITO/organic blend shows that the

Figure 10. Cross-section of a glass/ITO/PEDOT:PSS/blend layer structure.

adherence between the ITO electrode and the organic film is very good, without any visible porosity. The thickness of the films can be directly measured, where the thickness of PEDOT:PSS is 30 nm. Furthermore, it can be estimated that the thickness of the ITO/PEDOT:PSS/blend layer is ∼100 nm. 3.4. Electrochemical Properties of As-Synthesized Organic Compound. The features of electrochemistry were measured with the cycle voltage (CV) system for the potential of the oxidization and reduction to understand the electron affinity (Ea) and the ionization potential (Ip) of the compound, so that the difficulty involved with charge injection can be evaluated. With CV measurement, it may be understood that if there is an oxidization peak in the range of positive potential to realize, the compound is capable of receiving electrical holes; in contrast, if there is a reduction peak in the range of negative potential to realize, the compound is capable of receiving electrons. Based on the formula derived by Brédas,27 it is known that the ionization potential and the oxidization potential have close relationships. In order to calculate the absolute energies of HOMO (highest occupied molecular orbital)−LUMO (lowest occupied molecular orbital) levels of CuPc-712, SQ-700, and Pery-C7, with respect to the vacuum level, the redox data were standardized to the ferrocen/ ferricenium couple, which has a calculated absolute energy of −4.6 eV.28,29 The LUMO energies of the CuPc-712, SQ-700, and Pery-C7 were calculated from cyclic voltammogram and UV−Vis absorption spectra, as resulted in Figures 6 and 11, and Table1, respectively. The optical band gap can be calculated with the electronic absorption band, namely, Eg (eV) =

Figure 11. Cycle voltage (CV) for (a) CuPc-712, (b) SQ-700, and (c) Pery-C7 at a rate of 0.1 V/s in TBAP/DMF.

Table 1. Opto-electrical Parameters of CuPc-712, SQ-700, and Pery-C7

hc λ

as-synthesized component

λ on set (nm)

HOMO (eV)

LUMO (eV)

band gap (eV)

CuPc-712 SQ-700 Pery-C7

712 700 470

−5.40 −4.98 −6.70

−366 −3.21 −4.06

1.74 1.77 2.64

3.5. Photovoltaic Performance of Organic Composites. One of the most promising avenues to organic solar cells is to increase the number of absorbed photons by matching the donor absorption spectrum more closely to the solar spectrum. This has been achieved with narrow-band-gap polymers (Eg < 1.8 eV),30 by using a blend of a conjugated organic compound and a sensitizing dye,31 or by using blends of several organic semiconductors with varying band gaps.32 Figure 12 shows the energy band diagram of SQ-700, CuPc-712, and Pery-C7 before and after blending. Based on ref 33, the HOMO and LUMO

where h = 6.626 × 10−34 J s and c = 3 × 1010 cm s−1 (1 eV = 1.602 × 10−19 J). In addition, the oxidization potentials of CuPc-712 and SQ-700 are 0.80 V and 0.38 V, respectively, while the reduction potential of Pery-C7 is −0.54 V. Therefore, it is known that HOMO = −5.4 eV and LUMO = −3.66 eV for CuPc-712; HOMO = −4.98 eV and LUMO = −3.21 eV for SQ-700; and HOMO = −6.7 eV and LUMO = −4.06 eV for Pery-C7, respectively. 3635



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remove the photogenerated electrons and holes out of the recombination ranges, and the early recombination of carriers is inhibited by the spatial separation of holes (on SQ-700+CuPc712) and electrons (on Pery-C7), which increases the lifetime of the carrier and, subsequently, improves the photoconductivity as well. The photoelectrical efficiency of phthalocyanine reported by the literature11,33 was low (estimated to be 0.006%). In the present study, the I−V characteristics of the organic cells have been measured, as displayed in Figure 13, by illumination of a solar simulator using AM1.5 conditions in air at ambient temperature in darkness. Taking SQ-700/Pery-C7 (1:4) (Figure 13a), CuPc-712/ Pery-C7 (1:4) (Figure 13b), and CuPc-712/SQ-700/Pery-C7 (1:1:4) (Figure 13c) as composites, the photoelectrical efficiency

Figure 12. Energy-band diagram for the solar cell structured by (a) ITO/PEDOT:PSS/SQ-700+Pery-C7 (1:4)/Al, (b) ITO/PEDOT:PSS/CuPc-712+Pery-C7 (1:4)/Al, and (c) ITO/PEDOT:PSS/ CuPc-712+SQ-700+Pery-C7 (1:1:4)/Al.

energy levels were calculated from the CV measurements and UV−Vis absorption spectra in this work. The absorption of different wavelengths of light induces the creation of an exciton in the compounds; also, the interfaces between them act as the dissociation cites, where the photo-induced excitons split into free charge carriers. Increasing the heterojunction interface is favorable for the photo-induced excitons to split into free charge carriers.34 Because of the existence of the interfacial potential barrier, as demonstrated by the built-in potential in the D/A heterojunction, it is difficult for the split electrons and holes to be close to each other, making it possible to leave holes in the donor phase of SQ-700 and CuPc-712, as well as electrons in the acceptor phase of Pery-C7. At the same time, the electrons and holes can be transported effectively from the Pery-C7 and SQ-700/CuPc-712 phases to the ITO and Al electrodes, respectively, resulting in higher photosensitivity in the composite made from SQ-700, CuPc-712, and Pery-C7. Given the larger interfacial area in p−n heterojunctions, the photosensitivity is higher. In other words, it may help to

Figure 13. I−V curve on the solar cell structured by (a) ITO/ PEDOT:PSS/SQ-700+Pery-C7 (1:4)/Al, (b) ITO/PEDOT:PSS/ CuPc-712+Pery-C7 (1:4)/Al, and (c) ITO/PEDOT:PSS/SQ700+CuPc-712+Pery-C7 (1:1:4)/Al. 3636



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Table 2. Photovoltaic Performances of Composites Made from (a) CuPc-712+Pery-C7 (1:4), (b) SQ-700+Pery-C7 (1:4), and (c) CuPc-712+SQ-700+Pery-C7 (1:1:4) composite (weight ratio)

short-circuit current density, Isc (mA/cm2)

Impp (mA)

open-circuit voltage, Voc (V)

Vmpp (V)

fill factor, FF

photoelectrical efficiency, η (%)

0.844 0.403 0.603

0.44 0.226 0.408

0.7 0.65 0.7

0.35 0.45 0.45

0.18 0.56 0.43

0.10 0.14 0.18

SQ-700+Pery-C7 (1:4) CuPc-712+Pery-C7 (1:4) CuPc-712+SQ700+Pery-C7 (1:1:4)

(2) Hiramoto, M.; Fujiwara, H.; Yokoyama, M. p−i−n Like Behavior in Three-Layered Organic Solar Cells Having a Co-Deposited Interlayer of Pigments. J. Appl. Phys. 1992, 72, 4203. (3) Zimmermann, B.; Glatthaar, M. Electroabsorption Studies of Organic Bulk Heterojunction Solar Cells. Thin Solid Films 2005, 493, 273. (4) Gebeyehu, D.; Maennig, B.; Drechsel, J.; Leo, K; Pfeiffer, M. Bulk Heterojunction Photovoltaic Devices Based on Donor−Acceptor Organic Small Molecule Blends. Solar Energy Mater. Solar Cells 2003, 79, 81. (5) Girotto, C.; Cheyns, D.; Aernouts, T.; Banishoeib, F.; Lutsen, L. Bulk Heterojunction Organic Solar Cells Based on Soluble Poly(thienylene vinylene) Derivatives. Org. Electron. 2008, 9, 740. (6) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency Approaching 100%. Nat. Photonics 2009, 3, 297. (7) Sun, O.; Dai, L.; Zhou, X.; Li, L.; Li, O. Bilayer- and Bulk Heterojunction Solar Cells Using Liquid Crystalline Porphyrins as Donors by Solution Processing. Appl. Phys. Lett. 2007, 91, 253505,1. (8) Castro, F.; Faes, A.; Geiger, T.; Graeff, C. F. O. On The Use of Cyanine Dyes as Low-Band Gap Materials in Bulk Heterojunction Photovoltaic Devices. Synth. Met. 2006, 156, 973. (9) Müller, M.; Robert, E.; Dinnebier, R. E.; Jansen, M.; Wiedemann, S.; Plüg, C. The Influence of Temperature, Additives and Polymorphic Form on The Kinetics of The Phase Transformations of Copper Phthalocyanine. Dyes Pigm. 2010, 85, 152. (10) Merritt, V. Y. Organic Photovoltaic Materials: Squarlium and Cyanine-TCNQ Dyes. IBM J. Res. Develop. 1978, 22, 353. (11) Derouiche, H.; Bernède, J. C.; L’Hyver, J. Optimization of the Properties of Bulk Heterojunctions Obtained by Co Evaporation of Zn-phthalocyanine/Perylene. Dyes Pigm. 2004, 63, 277. (12) Wolleb, H.; Wolleb, A.; Schmidhalter, B.; Budry, J.-L. Metallocenyl Phthalocyanine. U.S. Patent 6,399,768, June 4, 2002. (13) Chen, Z.; Xia, C.; Wu, Y.; Zuo, X.; Song, Y. Synthesis Characterization and Third-Order Nonlinear Optical Properties of Bromo [Tri-α-(2,4-dimethyl-3-pentyloxy)subphthalocyanine]boron Complex. Inorg. Chem. Commun. 2006, 9, 187. (14) Ö zkaya, A. R.; Hamuryudan, E.; Bayir, Z. A.; Bekaroglu, Ö . Electrochemical Properties of Octakis (Hydroxyethylthio) Substituted Phthalocyanines. J. Porphyrins Phthalocyanines 2000, 4, 689. (15) Kim, S. H.; Hwang, S. H. Synthesis and Photostability of Functional Squarylium Dyes. Dyes Pigm. 1997, 35, 111. (16) Terpetsching, E.; Lakowicz, J. R. Synthesis and Characterization of Unsymmetrical Squaraines: A New Class of Cyanine Dyes. Dyes Pigm. 1993, 21, 227. (17) Kim, S. H.; Hwang, S. H.; Kim, N. K.; Kim, J. W. Aggregation and Photo Fading Behaviors of Unsymmetrical Squarylium Dyes Containing a Quinolylidene Moiety. J. Soc. Dyes Color. 2000, 116, 126. (18) Zhang, X.; Wu, Y.; Li, J.; Li, F.; Li, M. Synthesis and Characterization of Perylene Tetracarboxylic Bisester Monoimide Derivatives. Dyes Pigm. 2008, 76, 810. (19) Mo, X.; Shi, M. M.; Huang, J. C.; Wang, M.; Chen, H. Z. Synthesis Aggregation and Photoconductive Properties of Alkoxycarbonyl Substituted Perylene. Dyes Pigm. 2008, 76, 236. (20) Asir, S.; Demir, A. S.; Icil, H. The Synthesis of Nove Unsymmetrical Substituted Chiral Naphthalene and Perylene Diimides: Photophysical, Electrochemical, Chiroptical and Intramolecular Charge Transfer Properties. Dyes Pigm. 2010, 84, 1.

(η), short-circuit current density (Isc), open-circuit voltage (Voc), and fill factor (FF) were as follows: for Figure 13a, η = 0.10%, Isc = 0.844 mA/cm2, Voc = 0.70 eV, and FF = 0.18; for Figure 13b, η = 0.14%, Isc = 0.43 mA/cm2, Voc = 0.65, and FF = 0.56; and for Figure 13c, η = 0.18%, Isc = 0.603 mA/cm2, Voc = 0.70 eV, and FF = 0.43. The improvement of the photovoltaic. performance by SQ-700 blending with CuPc-712 can be attributed not only to the expansion of the light absorption spectrum but also to the presence of bulk heterojunctions. The contribution of the expansion of the light absorption spectra is clearly demonstrated by the good correlation between the optimal efficiency and the absorption spectra of the blend. After photon absorption and exciton creation, the exciton should be dissociated at a large D/A interface area. This large interface area is created by the blend structure. As listed in Table 2, the presence of a thin PEDOT:PSS film at the ITO/(CuPc-712+ SQ-700) interface allows one to achieve a significant solar cell efficiency of ∼0.2% in this work.

4. CONCLUSION The solubility and compatibility of squaraine dye, CuPc, and perylene derivatives were improved greatly after being attached to functional groups by covalent bonds, such that the composites with SQ-700, CuPc-712, and Pery-C7 could be prepared via the solution-blending method. Because of the interaction of the SQ-700, CuPc-712, and Pery-C7 molecules, a bulk heterojunction structure could be formed in the composite film with a suitable content ratio, leading to enhanced photosensitivity in the photoreceptor, which was interpreted in terms of the expansion of light absorption spectra and charge separation by the bulk heterojunctions. In addition, the presence of a thin PEDOT:PSS film at the ITO/(SQ-700+ CuPc-712+Pery-C7) interface allows one to achieve a significant solar cell efficiency of ∼0.2%. This is an initial study, but the results will be used for the future design of photoconductive or photovoltaic devices that have a high efficiency of charge carrier generation but require simple processing with low cost.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support by Material and Chemical Research Laboratory, Industrial Technology of Research Institute, Taiwan, ROC.

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REFERENCES

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